Will a 70 watt ac adapter work on a 90 watt laptop?

It will work but the center part that black box on the cord will start over heating and could cause the laptop to slow down or even eventually give it no power due to too much stress. Temporarily it should be fine it would take awhile for a new cord to start reacting funny but to be safe i would go with the 90 watt or higher but voltage is more important check the voltage if its plus or minus more than 10volts that is recommended don’t use itIt may work under some special conditions:1)You do not put pressure on you laptop which may lead to more energy consuming which may result in the failure of the adaptor in providing enough power.2)The voltage between + and - poles of the adapter must be similar to those of the original adaptor which came with the laptop.** A 10 to 20 percent difference may not cause any problem.3)The amount of current provided by the adaptor (in Amps) should have the same conditions, means that it shouldn't vary from those of the original adaptor more than 10 to 20 percents.Wattage is less of a problem. Polarity and Voltage is s big issue. If you don't know exactly, don't do it. You may damage a good machine.